1. Field of Endeavor
The present invention relates to the field of yttria-based thermal barrier coatings and their protection from calcia-alumina-magnesia silicate (CMAS) attack.
2. Brief Description of the Related Art
A thermal barrier coating (TBC) can be destabilized by environmental contaminants, mostly by calcia-alumina-magnesia silicates (Ca—Mg—Al—SiO, also called CMAS; note that S stands for SiO and not for sulfur) forming a deposit that can react with yttria-stabilized zirconia and lead to the TBC spallation. For a general description of the corresponding mechanisms as perceived in the state-of-the-art, reference is for example made to the introductory portion of U.S. Pat. No. 5,660,885. In recent years several solutions have been proposed in this respect.
U.S. Pat. No. 5,660,885—Protection of thermal barrier coating by a sacrificial surface coating: discloses a sacrificial oxide coating on a thermal barrier coating in an effective amount so that the oxide coating reacts chemically and is consumed by the contaminant composition at an operating temperature of the thermal barrier coating, by raising the melting temperature or viscosity of the contaminant composition, where the sacrificial coating is selected from the group consisting of alumina, magnesia, chromia, calcia, calcium zirconate, scandia, silica, magnesium aluminum oxide, and mixtures thereof.
U.S. Pat. No. 5,773,141—Protected thermal barrier coating composite: discloses a continuous sacrificial oxide coating adjacent to an outer surface of the thermal barrier coating, where the thermal barrier coating is a chemically stabilized zirconia selected from the group consisting of yttria-stabilized zirconia, scandia-stabilized zirconia, calcia-stabilized zirconia, and magnesia-stabilized zirconia, where the sacrificial oxide coating is about 0.2-250 micrometers thick, where the sacrificial oxide coating is not mixed with zirconia, and where the sacrificial coating reacts with contaminant compositions to prevent contaminant infiltration into the thermal barrier coating.
U.S. Pat. No. 5,851,678—Composite thermal barrier coating with impermeable coating: discloses a continuous impermeable barrier coating adjacent to an outer surface of the thermal barrier coating, where the impermeable barrier coating is dense and non-porous and where the impermeable barrier coating is about 0.2-250 micrometers thick, and where the impermeable barrier coating is selected from the group consisting of tantalum oxide, scandium oxide, hafnium oxide, magnesium aluminum oxide, calcium zirconate, silicon carbide, tantalum carbide, titanium carbide, tungsten carbide, silicon oxycarbide, silicon nitride, zirconium nitride, tantalum nitride, boron nitride, chromium silicide, molybdenum silicide, tantalum silicide, titanium silicide, tungsten silicide, zirconium silicde, silver, gold, and alloys thereof.
U.S. Pat. No. 5,871,820—Protection of thermal barrier coating with an impermeable barrier coating: discloses an impermeable coating for protecting TBC (thermal barrier coating) from liquid contaminants and the “impermeable barrier coating is selected from the group consisting of a metal oxide, a metal carbide, a metal nitride, a metal silicide, and mixtures thereof, where the metal oxide coating is selected from the group consisting of silicon oxide, tantalum oxide, scandium oxide, hafnium oxide, magnesium aluminum oxide, and mixtures thereof, where the metal carbide coating is selected from the group consisting of silicon carbide, tantalum carbide, titanium carbide, tungsten carbide, silicon oxycarbide, and mixtures thereof, where the metal nitride coating is selected from the group consisting of silicon nitride, zirconium nitride, tantalum nitride, boron nitride, and mixtures thereof, where the metal silicide coating is selected from the group consisting of chromium silicide, molybdenum silicide, tantalum silicide, titanium silicide, tungsten silicide, zirconium silicide, and mixtures thereof.”
U.S. Pat. No. 5,914,189—Protected thermal barrier coating composite with multiple coatings: discloses thermal barrier coated parts which have least two outer protective coatings that decrease infiltration of molten contaminant eutectic mixtures into openings in the thermal barrier coating.
U.S. Pat. No. 6,261,643—Protected thermal barrier coating composite with multiple coatings: discloses a system having two protective layers prepared by depositing an impermeable barrier or sacrificial oxide first coating on the thermal barrier coating, and then depositing at least one other protective second coating that is non-wetting, sacrificial or impermeable on the first coating.
U.S. Pat. No. 6,627,323—Thermal barrier coating resistant to deposits and coating method therefore: discloses “a protective coating system overlying the thermal barrier coating, the protective coating system comprising inner and outer alumina layers and a platinum-group metal layer encased therebetween.”
U.S. Pat. No. 6,720,038—Method of forming a coating resistant to deposits and coating formed thereby: discloses a method of forming by PVD a coating containing a mixture of yttria-stabilized zirconia and of alumina that should prevent CMAS infiltration.
U.S. Pat. No. 6,794,059—Multilayer thermal barrier coatings: discloses a porous non-sacrificial top coat for protection against environmental contaminants, in which the top coat is a material selected from the group consisting of carts stabilized zirconia (CSZ); calcium-stabilized zirconia (CaSZ); zirconia toughened alumina (ZTA); a compound oxide comprising two or more compounds from the group consisting of oxides of aluminum, cobalt, chromium, iron, titanium and nickel and mixtures of two or more thereof; and the top coat is selected from materials more resistant to the environmental contaminants than the thermal barrier coating.
U.S. Pat. No. 7,226,668—Thermal barrier coating containing reactive protective materials and method for preparing same: discloses a multilayer system having an outer layer protecting from CMAS attack composed of (1) a CMAS-reactive material in an amount up to 100% and sufficient to protect the thermal barrier coating at least partially against CMAS that becomes deposited on the exposed surface, the CMAS-reactive material having an alkaline earth aluminate, alkaline earth aluminosilicate or mixture thereof, wherein the alkaline earth is selected from the group consisting of barium, strontium and mixtures thereof; and (2) optionally a ceramic thermal barrier coating material.
U.S. Patent Application Publication No. 2006/280955 discloses an article having a silicon-containing substrate, an environmental barrier coating (EBC) overlying the substrate, wherein the EBC includes an outer alkaline earth aluminosilicate barrier layer; and a corrosion resistant alumina/aluminate sealant for the outer barrier layer. A process is also provided for forming a corrosion resistant alumina/aluminate sealant layer over the outer barrier layer of the EBC. Also provided is an alternative process for treating a porous outer barrier layer with a liquid composition including a corrosion resistant alumina/aluminate sealant precursor to infiltrate the porous outer barrier layer with the alumina/aluminate sealant precursor in an amount sufficient to provide, when converted to the corrosion resistant alumina/aluminate sealant, protection of the environmental barrier coating against environmental attack; and converting the infiltrated alumina/aluminate sealant precursor within the porous outer barrier layer to the corrosion resistant alumina/aluminate sealant. The document clearly states that sulfates are corrosive species and in many text portions mentions ‘sulfates and/or chlorides of calcium, magnesium, sodium, etc.,’ which includes CaSO4 as a corrosive species.
Most of these documents disclose materials that should react with the liquid deposit forming a solid phase that should not infiltrate the TBC. The other documents describe impermeable coatings that should prevent the liquid infiltration.